Information on the in situ state of stress in the earth's crust is important to the proper analysis, design and construction of underground openings such as those for mining operations and for underground civilian and military installations. The suitability of a particular region of rock deep underground to such installations is significantly dependent upon the state of stress since the stress on the rock can greatly affect the stability of the rock mass. In addition, knowledge of the in situ state of stress can become critical in other applications, such as petroleum and gas extraction, the exploitation of geothermal energy from hot subsurface rocks, and the development of new methods of in situ exploitation of mineral and energy resources, as well as in earthquake studies where a thorough understanding is desired of the mechanisms of active faults and crustal strains.
Stress, a fictitious quantity, cannot be measured directly. Thus, the manifestations of stress are measured and used to estimate the stress components. For example, effects of stress that have been used or proposed for use in estimating stress in deep rock include the effect of rock stress on the velocity of sound waves, the increased secondary gamma radiation intensity absorbed with increased stress loading, and various types of strain relief methods. Estimating stress by examining the level of strain within the rock has the advantage that the strain has the same number of components as the stress and, for elastic materials, the stress and strain are directly related. Strain measurement to determine stress may be carried out by either strain relieving or strain compensation methods. The strain relieving method involves the relief of the original stress and the measurement of the deformation associated with the relieving operation. In the strain compensation method, the original stress is disturbed and the restoring pressure is measured.
Many of the currently used methods and associated devices for measuring in situ stresses were developed for mining and civil engineering applications. The method commonly utilized today for in situ stress measurements in deep rocks is hydrofracturing, which was developed to enhance production in the gas and oil industries. Hydraulic fracturing or hydrofracturing yields an average estimate of the secondary principal stresses in the plane perpendicular to the axis of the borehole. The axial principal stress in the direction of the borehole must be estimated on theoretical grounds as the weight of the superincumbent rock mass. This method, however, requires that the borehole lie along a principal stress direction. If only a single borehole is used in the measurement, the method becomes ambiguous when lateral stresses are high compared with axial stresses. Observations in the field have also indicated that there may be some inaccuracies in the magnitude and orientation of the measured stresses.
At present, hydrofracturing appears to be the only method capable of being carried out at great depths. The overcoring method is also capable of yielding reasonably accurate measurements of stress, but is not suitable to measuring at great depth. In the overcoring method, an initial small pilot hole is drilled at the bottom of a borehole and deformation gages or strain rosettes are applied to the wall of the small pilot hole. A larger diameter borehole is then drilled, using a tubular drill, around the pilot hole to release the strain on the core of rock which surrounds the strain measurement devices in the pilot hole. The changes in dimensions of the core reveals the initial state of strain on the core which was released by the overcoring operation, which can then be used to estimate the initial level of stress in the region of the core. A principal limitation of the overcoring technique is that it is difficult to perform in deep holes, in part because the wires connected to the sensors extend through rotating drill pipe and are subject to tangling as the pipe rotates. A number of other limitations of the overcoring method have also been noted. It must, of course, be performed at the bottom of a drilled hole, eliminating the possibility of using existing holes which are deeper than the depth at which tests are desired. In addition, since the original drilling of the hole released the vertical pressure on the rock directly beneath the bottom of the hole, the stress field measured by the overcoring technique is primarily that which is substantially perpendicular to the axis of the hole. Thus the readings obtained by the overcoring technique may not be accurately related to the actual state of stress in the surrounding rock. The overcoring technique is also relatively expensive since it requires an entire drill rig and crew. Even where tests are to be performed on a preexisting hole, a drill rig must be brought to the site of the hole so that the overcoring operation can be performed at the bottom of the preexisting hole.
Other, more recently developed methods for measuring stress are the borehole deepening method, sidehole overcoring method and jack fracturing method.
In the borehole deepening method, the diametral deformations of the wall close to the bottom of the borehole are measured while the hole is being deepened with a specially designed tapered drill bit, as illustrated in U.S. Pat. No. 3,538,755. The method has been successful in near surface and tunnel-driving measurements, but encounters technical difficulties in measurements in deeper holes. In the sidehole drilling method, the borehole walls are first instrumented with electrical resistance strain gauges. Small holes are then drilled over or under the strain gauges in the borehole wall to relieve the stresses. Both the relieving process and the strain-relief measuring process are difficult to carry out with this method, especially at great depths and in severe borehole environments. In the jack fracturing method, as shown in U.S. Pat. No. 3,961,524, friction strain gauge rosettes are first applied to the borehole wall. The wall adjacent the strain gauges is then fractured by unidirectional loading with a borehole jack. With the fractures kept open under pressure, the stresses in the rock adjacent the wall are relieved and the change in the borehole wall dimensions can be detected by the strain gauges. The in situ state of stress can then be calculated from the strain gauge signals using elastic theory. While this method has been used successfully at shallow depths, its performance at great depths has not yet been established.